The invention relates to a display tube comprising an envelope including a display window having on its inside surface a display screen; a transparent, substantially parallel, second window provided in parallel spaced relationship to the outside surface of the display window; and a pump, the space between the display window and the second window being destined for a flow of a translucent cooling liquid stimulated by the pump.
The invention also relates to a colour television projection device comprising such a display tube.
A display tube as mentioned hereinbefore is known from EP-A-No. 162972. The display screen of such a display tube usually comprises a phosphor layer on which a frame is written by means of an electron beam. As a result of the electron bombardment the temperature of the display screen rises so that the luminous efficiency of the display screen decreases. This effect is termed thermal quenching. This is the case in particular in display tubes for projection television, in which, in order to obtain the required high brightnesses, the display screens are scanned by electron beams with large beam currents. In order to check the decrease of the luminous efficiency as described in EP-A-No. 162972, the display window and the display screen disposed thereon are cooled by causing the cooling liquid to flow through the space between the display window and the second window by means of a pump driven by an electric motor. Since the motor is present at a very short distance from the display tube, the operating motor may adversely influence the path of the electron beams in the display tube due to its electromagnetic properties, which may for example give rise to frame errors.
It is an object of the invention to provide a display tube which is cooled by pumped flow of a cooling liquid through the space between the display window and the second window and in which no detrimental influences are exerted on the operation of the display tube.